1. Field of the Invention
The invention relates to development of latent images in an electrostatic printing process and electrophotographic process and, in particular, relates to a development process using conductive particles.
2. Discussion of Related Art
Electrophotography was invented in 1938 by Chester Floyd Carlson and spread throughout the world because of the superior resolution and output speed. The field of electrophotography has been constantly improving and improvements continue today. One example of electrophotography is electrostatic printing. The material used in developing an electrostatic latent image is referred to as toner and various improvements to toner have been a large factor in the world-wide growth of copying machines and printers.
In general, toner is a mixture of pigment and resin particles. Commonly, an electrostatic latent image is developed using an electrostatic charge with a carrier or development sleeve (in a dry system) or solvation (in a liquid system). The toner holds an electrostatic charge and is transported to the vicinity of the electrostatic image holding body where the latent image is developed when the toner is attached to the image holding body by an electric field. For a system requiring transfer of the latent image (i.e., copiers and printers), the toner maintains an electrostatic charge on the image holding body and it is transported onto the transfer media by an electric field of opposite polarity to the toner charge at a transfer station.
In order for the electrophotographic process to function as presently practiced the toner's surface must be an insulating material that holds an electrostatic charge. In recent years, various improvements have been made that have led to the development of conductive toners that meet the requirements of manufacturing a printed circuit board directly from digital data taking advantage of the high resolution and fast output speed characteristic of electrostatic printing.
Existing systems and methods are utilizing metal (e.g., conductive) toner by covering the conductive particles with an insulating resin. These prior systems and methods aim for a development process in electrophotography using metal particles covered with a thin insulation membrane so that the insulated metal particles behave similar to an insulated toner with respect to electrostatic imaging processes. In these existing electrostatic printing and electrophotographic processes, electrically conductive toner and metal toner are covered with an insulation layer as discussed above—they are not simply conductive particles and metals.
It is well-known and relatively easy to make an insulating membrane layer on conductive material for use with presently known electrostatic printing or electrophotographic process, but it is very difficult to use conductive material made without such an insulating membrane in presently known electrophotographic printing processes. The purpose of the conductive material in printed traces of a printed circuit board is to apply an electrical current through the conductive material, but insulating the conductive material with a membrane makes the resulting printed traces high in resistance, be it an electrode or wiring. Present methods require removing the insulation membrane layer with high temperature to purify the conductive material imprinted as electrical traces. For that reason, use of electrophotography for producing printed circuit board is currently only in limited applications.
In order to improve the conductivity, another approach as shown in Japanese patent disclosures No. 1983-57783 and No. 1995-254768 includes vanadium on the surface of regular toner. Vanadium is the base material used in well known metal plating processes. After patterning and fixing the vanadium image traces on a circuit board, a metal plating process is done to form a conductive pattern. However, the merit of manufacturing a printed circuit board directly from digital data is diminished by adding the extra process. Also, industry requirements that no impurities be added to the conductive material of a printed circuit board are becoming more critical. It is more important that the conductive material of a printed circuit board be pure and highly conductive.
At present, a pure conductive material (e.g., un-insulated metal) cannot be used by electrophotographic and electrostatic printing processes without pre-treatment because all the required characteristics for electrostatic toner are contrary to the characteristics of a conductive material. The toner requirements are in general as follows:                it must be dispersed as single particles;        it needs to be transported to the vicinity of an image holding body;        it cannot scatter while transported;        it must not destroy the electrostatic latent image;        it has to be charged with the correct electrostatic polarity characteristics; and        it must not attach to any parts other then the targeted latent image.        
Regular (e.g., non-conductive) toner is almost an insulator and the each particle repels from each other when it is charged with static electricity. Also, toner attaches to other materials, called carrier, through charging and does not scatter, even when transported mechanically. Even more particularly, toner will not destroy a latent image it touches since toner is an insulator and toner will only attach to the location where it is electrostatically attracted and will not attach where repelled. Standard, known, non-conductive toner is made so that each particle is evenly well charged and made with a certain intended polarity.
However, it is difficult to disperse and charge individual particles of conductive material and difficult to maintain the electrostatic charge. Because of the conductive properties of the charged particles, they would scatter when transported and destroy any latent image they may come in contact with.
It is therefore evident from the above discussion that a need exists for improved apparatus and methods for developing an electrostatic latent image using conductive particles.